I agree, do not release them outside. That is a very bad idea.
You will have to smash them or use bleach, but I think smashing them would be more humane, I think the die instantly that way....
You can keep him, but those snails reproduce like crazy. I had a problem with them and found them very hard to get rid of, especially because I did not want to kill them..
They are pretty cool though, and if you keep the food under control they won't reproduce as often, but you will still find yourself daily checking closely for eggs to get rid of.